We all know trees provide shading and they also cool the local
environment by evapotranspiration. However, scientists have long
debated the role of plant transpiration on global climate. The
reason is that while evaporation causes a local cooling, condensation
of the same water heats up the atmosphere somewhere higher but within
the climate system. Globally, this cycle of evaporation and condensation
moves energy around, but cannot create or destroy energy. How could
then evapotranspiration change global climate if the net heating is
zero?

According to a new study by scientists at the Indian Institute of
Science, the Carnegie Institution for Science at Stanford University
and Lawrence Berkeley National Laboratory, evaporated water helps
cool the earth as a whole, not just the local area of evaporation,
demonstrating that evaporation of water from trees and irrigated crop
areas could cool the planet. These findings, published online on 14
Sept 2011 in Environmental Research Letters, could have major
implications for land-use decision making.

It is well known that clearing of forests for agriculture and infrastructure
development can contribute to local warming by decreasing local evaporative
cooling, but it was not understood whether this decreased evaporation would
also contribute to global warming.

When there is no net heat added to the planet, how does a change in
surface evapotranspiration cause planetary cooling? “It is the
feedback loops in the climate system” says Prof. G, Bala of the
Indian Institute of Science in Bangalore. “In this case, it is
primarily the cloud feedback. Enhanced surface evaporation causes
an increase in the amount of low level clouds in the atmosphere.
These clouds scatter more solar radiation back to space and cool
the planet”.

In the past, many climate modeling studies on tropical deforestation
studies have indeed simulated the local warming from deforestation.
But because water vapour plays so many roles in the climate system,
the global climate effects of changes in evaporation were not well
understood. The researchers even thought it was possible that
evaporation could have a warming effect on global climate, because
water vapour acts as a greenhouse gas in the atmosphere.

“The policy implications of this study are enormous. The recent Food
and Agriculture Organization estimate shows that around 13 Million
hectares of forests are converted to other uses or lost each year.
In addition to contributing to global warming through CO2 emissions,
deforestation in the tropics could be causing more global mean
warming through reduced evapotranspirtation” says Prof. Bala.